I tried to look at the actual GeoServer documentation but I could not find specifics about how a shared data directory can be used, is it supported, which features work with it and which dont. Best I could find was "When setting up a cluster of GeoServer machines it is common to share a single data directory among all the cluster nodes" at http://docs.geoserver.org/latest/en/user/advanced/logging.html.

It is not a given that a shared data directory can be used. The nodes could be writing all sorts of files in the data dir, blocking and interfering each other. Without clear GeoServer documentation about it I am hesitant to use this approach in production...